A lot depends on your goals, budget and your abilities.
The Duramax/Allison is by far the best performing, highest tech alternative (high on the 'cool factor' too) and possibly one of the lighter combinations since it has aluminum heads, where as the absolute simplest would be to bolt in an old 'Toroflow' V6 GMC diesel in place of the original (what was there, V6?) gasoline engine. It is nearly a bolt in swap from the gas V6. BUT... This is an OLD engine with very old technology. Doesn't have much power, doesn't give very good fuel economy.

Between these two extremes are a miriad of diesel engines with a wide range of complexity, weight and performance

I have seen complete Duramax engine/transmission/electronics setups for sale that seem perfect for a conversion... Just would need to look hard for it.